This invention relates to indirect frequency modulators and more specifically to those indirect frequency modulators using phase locked loop techniques.
Phase locked loop techniques have been known since the early 1930's but had been somewhat avoided because of their high cost and complexity in discrete system design. With the developments in integrated circuit design and processing this situation is rapidly changing and phase locked loops are becoming versatile building blocks with many applications.
Basically a phase locked loop is a frequency feedback system comprised of a phase detector, a low-pass filter and a voltage controlled oscillator (VCO) with a feedback path. When the input signal to the loop is zero the voltage controlled oscillator operates at a predetermined free-running frequency. If an input signal is applied, usually by a crystal controlled oscillator, the phase detector compares the phase and frequency of the input signal with that of the voltage controlled oscillator and generates an error voltage that is related to the difference in the two signals. The error voltage is then filtered and applied to the control of the VCO, thereby varying the VCO frequency in a direction that reduces the frequency difference between the two signals. When the frequencies of the two signals become sufficiently close, the feedback nature of the system causes the system to lock with the incoming signal. Once in lock the VCO frequency is identical with the input signal, except for a finite phase difference which is necessary to generate the corrective error voltage to shift the VCO frequency to the input signal frequency, thus keeping the system in lock. The range of frequencies over which the loop can acquire lock with an incoming signal is called the "capture range." The range of frequencies over which the loop can maintain lock is called the "lock range" and is broader than the "capture range."
Used as a building block, the phase locked loop is suitable for a wide variety of applications including FM demodulation, frequency synchronization, signal conditioning and phase modulation.
In the art there is known to be two general methods for generating frequency modulated signals, namely, indirect FM and direct FM.
In the indirect method of producing frequency modulation, the modulating signal is first integrated and then phase modulates a crystal controlled oscillator signal to produce a narrow-band FM signal. Frequency multiplication is then used to increase the resulting frequency deviation to the desired level.
Direct FM systems are those in which the modulating signal directly controls the carrier frequency. In this system, the transmitter output frequency may be modulated directly without frequency multiplication, but there is a discriminator circuit needed for stabilization of the center frequency. In contrast, the indirect method requires a number of frequency multipliers to product wideband FM, but does not require added circuitry for carrier frequency stabilization.
Heretofore phase locked loops have been used for phase modulation, these systems being suitable building blocks for indirect frequency modulating systems. However, since frequency multipliers are needed in such a system to achieve adequate frequency deviation and such frequency multipliers consist of tuned circuits, full integration of such a system is not practical.